Car performance improvement

jimmy_wood

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I have just had the oxygen and fuel level sensors replaced on my 8 year old CLK320 after both packed in immediately after a catalytic convertor replacement (strange and suspicious but not the subject of this post). The point is that I have noticed a marked improvment in acceleration performance.

Is this normal and how can this be?
 

wright

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it's probably because your old cat was probably breaking down and blocking the exhaust and now it can flow properly the engine is running correctly.
 
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jimmy_wood

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Not sure about that

There was a 3-4 week lag between getting cat done and the new sensors. Its only since sensors were fitted that performance improvement has come about.

I was looking at an exhaust diagram on what looks like a Russian parts site (link found in these forums) and noticed there are 2 oxygen sensons with one feeding back to the engine. I am assuming therefore that the new sensor is controlling the air/fuel mixture better?
 

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Yes correct,,the O2 sensors job is to monitor the gasses that pass through the engine,,should they become coated on contaminated they will weaken the mixture.
 
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jimmy_wood

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I thought the oxygen sensor was just an emmissions thing. I will have to check which oxygen sensor they actually replaced. Would either or both of them cause the warning light on the dash to come on?

On second thoughts improved performance might have come about partly due to cat replacement as well ... as the preformance wasnt this good before dash warning came on and the sensors were replaced ie when they were working.

Its not that the car was sluggish before ... but now it is noticably better. It wasnt bad enough that services picked anything up either.

CAT was pricey enough and have still to see the sensor bill.
 

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I thought the oxygen sensor was just an emmissions thing. I will have to check which oxygen sensor they actually replaced. Would either or both of them cause the warning light on the dash to come on?

On second thoughts improved performance might have come about partly due to cat replacement as well ... as the preformance wasnt this good before dash warning came on and the sensors were replaced ie when they were working.

Its not that the car was sluggish before ... but now it is noticably better. It wasnt bad enough that services picked anything up either.

CAT was pricey enough and have still to see the sensor bill.

Yes they are there for the emissions, and adjust the fuel air ratio accordingly . They have a certain range where they can adjust the mixture,,if they shift too far, then this will put the lamp on to warn that something has happened or taken place
 

124coupe

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Hi, if your car has two oxygen sensors then they work like this:

The first one (before the CAT) is used by the ECU to trim fuel/air ratios (to allow fine control and allow for tolerances in the air intake and fuel systems as they wear or get dirty).

If this sensor wears or gets fouled, the car will run poorly (and may have contributed to your CAT's death by providing it with a too-rich mixture and so overheating it) until/unless the ECU finally decides the sensor is "lying" by outputting an "implausible" signal and so a) puts the check engine light on and b) runs on a "safe" mixture map - this WILL impact performance.

The second one is after the CAT and is used to check if the CAT is alive and working. THe ECU compares values from the first and second sensors and looks for a difference - i.e. that the CAT is still burning pollutants and therefore "using up" oxygen from the exhaust gasses.

If there is little/no difference, then the light comes on, this time for emissions purposes.

Its always a good idea to get a car checked over before fitting a replacement CAT as the new one can be "killed" (contaminated or overheated) by the same fault as the old....
 
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jimmy_wood

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Thanks for this very informative reply.

The CAT needed replaced because the internals started to break up and the rattle was unbearable. The warning light didnt actually come on at this stage and I assume the emmissions were ok.

It was only after the CAT replacement that the check engine warning light came on and the fuel gauge stuck at full.

I asked for an emmissions check before agreeing to the O2 sensor change at they were fine (so new CAT was working).

My assumption was that the CAT replacement damaged these sensors ... then again maybe it was just coincidence? Bottom line is that I am paying for them!
 

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The lamp can come on if the O2 sensor moves either way to much from its stored setting, during the fuel contamination thing when the oxygen sensors were coated in silicon, they in turn weakened the mixture, Most MB drove OK on this weak mixture,,on small engined cars they would not run as the fuel was too weak,so if it gets fouled it will weaken the mixture, not over fuel it
 

124coupe

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The lamp can come on if the O2 sensor moves either way to much from its stored setting, during the fuel contamination thing when the oxygen sensors were coated in silicon, they in turn weakened the mixture, Most MB drove OK on this weak mixture,,on small engined cars they would not run as the fuel was too weak,so if it gets fouled it will weaken the mixture, not over fuel it

Agreed for contamination...

"Old age" in a heated O2 sensor is caused by gradual loss of the catalyst molecules and therefore an increasing inability to generate full design voltage - hence a false "lean" report leading to enrichment by the ECU and consequent increased temps in the catalytic converter (so says Bosch in their PDF).
 

124coupe

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Also, the tech "whacking" the exhaust to free off the old cat might have been fatal to your old o2 sensor - the ceramic "nose" might have cracked off!

Can't see any link with the fuel level sensor in the tank though (unless the hammer slipped!).
 

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Agreed for contamination...

"Old age" in a heated O2 sensor is caused by gradual loss of the catalyst molecules and therefore an increasing inability to generate full design voltage - hence a false "lean" report leading to enrichment by the ECU and consequent increased temps in the catalytic converter (so says Bosch in their PDF).

Yes I will go along with that completely ,we had a big article in the trade press last month
 
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jimmy_wood

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Probably damaged in the case of the O2 sensor and coincidence in relation to the fuel level gauge.

Ah well I will soon see what the bill is for their replacement when it comes in!
 
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